The present invention relates to the field of displacement sensing, and in particular to capacitance measurement based displacement sensing, for example to detect when an object, such as a user's finger, presses on a moveable surface.
Capacitive sensing techniques have become widespread for providing touch-sensitive inputs, for example in computer tablets, mobile phones, and in many other applications. Touch sensitive input devices are generally perceived to be more aesthetically pleasing than input devices that are based on mechanical switches. Nonetheless, the present inventors have recognised there are still situations in which a user-interface that is responsive to mechanical input may be desired. In particular, the inventors have recognised there are situations in which there is a desire to measure the physical displacement of a displacement element, for example to provide the equivalent of a “click” when navigating a cursor across a display screen using a touch sensor. Furthermore, the inventors have recognised it can be desirable to provide such functionality using capacitive sensing techniques rather than mechanical switching techniques. Not only can capacitive sensing techniques provide for more reliable sensors (as they are less prone to mechanical wear), there may be situations in which displacement sensing is desired in conjunction with other sensors based on capacitive sensing (for example to measure the displacement of a capacitive touch screen), and so it can be convenient to adopt the same sensing technologies for both touch position and displacement sensing aspects.
One issue with using capacitive techniques for sensing the displacement of an element is the potential for the presence of whatever is causing the displacement, such as a user's finger, to affect the capacitance measurements in addition to the effect of the displacement itself. Another issue with known displacement sensors using capacitive sensing is a relatively high dependence on manufacturing tolerances in what can be relatively complex configurations. This is because relatively small changes in a gap between two conductors can lead to a relatively high change in their capacitive coupling (because of the reciprocal dependence on separation).
There is therefore a desire for apparatus and methods for sensing the displacement of an object relative to another object using capacitive sensing techniques which help to address some of these issues.